Charge-discharge cycle performance of lithium-ion batteries at room temperature
At room temperature, how does a lithium-ion battery perform after a certain period of charging and discharging? This is a direction for improvement in lithium-ion battery technology, which requires the application of certain test parameters. Because the popularization of new energy vehicles in my country is accelerating, collecting test data from large-capacity lithium-ion batteries helps to understand the performance and characteristics of power lithium-ion batteries.
Based on tests of lithium-ion batteries, the following general conclusions can be drawn: During constant current and constant voltage charging phases, the ratio of constant current charging capacity to total charging capacity decreases with increasing cycle count; the discharge capacity of the 3.7V~4.2V discharge plateau accounts for over 90% of the total discharge capacity, and the charge/discharge efficiency is unaffected by the number of cycles. A detailed description follows.
Before describing the data, the test environment should be explained: BYD 80Ah lithium cobalt oxide battery was selected for charge and discharge test at room temperature (10℃~250℃).
Charge/discharge system design: Charging is constant current and constant voltage. First, charge to 4.2V at 1C or 80A constant current. 2. After 10 minutes, charge to 2.75V at 80A constant current. 3. After continuous discharging for 10 minutes, a new charge/discharge cycle is started, repeated 500 times.
During this process, relevant data needs to be collected to form corresponding charts: 2.2. The relationship between the proportion of constant current charging capacity to total charging capacity and the number of cycles; 3. Discharge curve; 4. Charge and discharge efficiency curve.
As can be seen from the above figure:
1. Starting from the constant current charging stage, the charging platform of lithium-ion batteries is 3.8V~4.1V, and the charging capacity in this stage accounts for more than 80% of the total charging capacity. As the number of cycles increases, the voltage rises faster, the charging time shortens, and the charging amount gradually decreases.
2. The percentage of constant current charging capacity in the total charging capacity decreases slightly with increasing cycle count, while the percentage of constant voltage charging capacity increases slightly. This indicates that with increasing lithium-ion battery charge-discharge cycles, a lower current results in better charging performance.
3. Based on the discharge curve, the discharge plateau (the discharge curve is stable within a certain voltage range, close to a straight line, rather than the distance between the previous rise and fall sloping lines) with the increase of the number of cycles, and the published discharge plateau of 4.2V~3.7V accounts for 90% of the total discharge capacity.
4. Charge/discharge efficiency: This refers to the percentage of discharged energy relative to the charged energy. It indicates the battery's discharge capacity. As seen from the charge/discharge efficiency curve, this value remains relatively constant, consistently above 99%.
We understand that the capacity of lithium-ion batteries decreases with increasing charge-discharge cycles, as shown in the data above. Specifically, this manifests as a reduced discharge plateau, shorter charging time, and a lower constant-current charging ratio. Ultimately, the rechargeable power decreases with each cycle, and the rate of decrease accelerates. After 500 cycles, the capacity must be at least 80% to be considered合格 (qualified/acceptable).
